Single-screw extruders are most widely used in processing plastic materials for melting solid plastic into molten state, or melt, suitable for forming into desired shapes. The performance of an extruder basically depends on the geometrical features of the screw. Among various types of special screws developed to improve the extruder performance, barrier screws utilizing a barrier flight have been most successful. The barrier flight with a tight clearance divides the screw channel into a solid channel and a melt channel. Only molten plastic material can flow over the barrier flight from the solid channel into the melt channel.
Although the barrier screws significantly improve the extruder performance, all of them have some undesirable structural features and their structures still can be improved. Problems encountered with previous barrier screws include, but are not limited to, blockage to solid bed movement, degradation of plastic material at dead-spot or in deep melt channel, small solid bed pieces entering melt channel from solid channel with a large depth at the end, overheating of melt at the end of solid channel with a very shallow depth at the end, poor metering capability of very deep melt channel at the end, and undesirable melt distribution from deep melt channel into shallow metering channel in the direction opposite to drag flow.